Because of their excellent mechanical characteristics, such as impact strength, and because of their excellent heat resistance and transparency, polycarbonates are widely used for various purposes.
Usually, polycarbonates are prepared by direct reaction of aromatic dihydroxy compounds such as bisphenol with phosgene (interfacial method) or by ester interchange reaction (polycondensation reaction) with carbonic acid diesters such as diphenyl carbonate.
In a process of preparing polycarbonates by ester interchange reaction of aromatic dihydroxy compounds with carbonic acid diesters, the reactants are allowed to react in a molten state by heating under reduced pressure usually in the presence of alkaline compounds used as alkaline catalysts. This process, therefore, has such an advantage that the desired polycarbonates can be prepared at a relatively low cost in comparison with the above-mentioned interfacial method.
Such alkaline catalysts as mentioned above are preferably those containing alkali metal compounds and/or alkaline earth metal compounds from the standpoint that the ester interchange reaction or polymerization reaction can proceed thereby at a sufficient rate.
However, we have discovered disadvantages resulting from the use of these alkaline catalysts. The alkali metal compounds and or alkaline earth metal compounds contained in the catalysts remain in the resulting polycarbonate in large amounts, and the remaining compounds adversely exert serious influences upon heat resistance, molding stablility (melt stability when molding), water resistance and weathering resistance inherent in polycarbonate.
On that account, it has been strongly desired the advent of a process for preparing aromatic polycarbonates excellent in heat resistance, water resistance and weathering resistance, and also stable even when allowed to stand in a molten state for an extended period of time in a case where the desired aromatic polycarbonates have been intended to obtain by melt polycondensation of aromatic dihydroxy compounds in alkaline compound catalysts with carbonic acid diesters. In addition, it has also been strongly desired the advent of an aromatic polycarbonate composition having the above-mentioned properties.